US health officials are investigating reports that Moderna Inc's Covid-19 vaccine may be linked to a higher risk of a rare heart condition in younger adults than previously thought, the Washington Post reported, citing people familiar with the review.

There might be a 2.5 times higher incidence of myocarditis in those who get the Moderna vaccine compared with Pfizer's vaccine, the Post quoted a source as saying.

The investigation that is focused on Canadian data suggests that risks of myocarditis might especially be higher for males below the age of 30 or so, according to the report.